In light of the recent global environmental problems, it is a target of attention to use biodegradable (microorganism-degradable or naturally degradable) materials in order to prevent environmental pollution caused by industrial waste. Recently, voluntary restraint of CO2 emissions is strongly demanded in order to deal with the exhaustion of earth resources and the global warming. In such a situation, naturally occurring materials as opposed to petroleum-derived materials, and materials which require a small amount of heat or emit a small amount of CO2 when being incinerated, are paid attention to.
It is conventionally known that polymers having an aliphatic ester structure are biodegradable. Representative examples of such polymers include poly-3-hydroxybutyrate (PHB) produced by microorganisms, polycaprolactone (PCL) which is a synthetic polymer, polybutylene succinate (PBS) or polybutylene succinate adipate (PBSA) each containing succinic acid and butanediol as main components, polyester carbonate (PEC), polylactic acid (PLA) obtained from L-lactic acid and/or D-lactic acid produced by fermentation as a main starting material, and the like. Among these, PLA, for example, is a naturally occurring material.
These polymers having an aliphatic ester structure, except for PLA, generally have properties similar to those of polyethylene and have good moldability and biodegradability. However, such polymers are not sufficiently strong in a field requiring rigidity or a field requiring tensile strength. The rigidity of these polymers may be improved using a filler such as talc or the like or using a nanocomposite forming technology. However, there are problems including reduction of fluidity, and improvement on this point has been desired. Regarding PLA, improvement in thermal resistance and toughness, and also hydrolysis resistance have been strongly desired.
There is no specific limitation on the polar thermoplastic resin. For example, polyacetal is a polymer which is of alphatic ether type or contains alphatic ether as a main component, is mainly derived from methanol that is a petroleum-independent raw material, and is considered to have a low environmental load. Polyacetal is used as an engineering plastic material having splendid mechanical properties, sliding properties, friction and abrasion resistance, chemical resistance and the like. Polyacetal is widely used for core components of automobiles, office automation devices, and the like. Polyacetal exhibits a high crystallinity because of the regular primary structure thereof, and the fields of use thereof have been expanding from the field of injection molding.
When a composite of polyacetal and another resin is formed, the high crystallinity of polyacetal is likely to act as an obstacle against the adhesiveness at the interface. A known successful example is merely formation of a polymer alloy material with polyurethane. In addition, since polyacetal has a functional group substantially only at a polymer terminus, the reactivity of a surface of the molded piece is low and so the adhesion strength of polyacetal with another resin and decorability are inferior. In order to improve the adhesiveness at the interface, it has been attempted to use various compatibility improving agents, perform corona discharge, ion etching, and the like, and even to modify the primary structure of the polymer. However, none of such attempts has sufficiently improved the adhesion strength at the interface despite the complicated operations.
The present inventors have produced a resin composition of a polymer having an aliphatic ester structure and a polymer having an oxymethylene structure, in which the polymers are highly compatible with each other, and also the crystallinity of the polymer having an oxymethylene structure in the resin composition is low. Such a resin composition expresses a sufficient biodegradability and has splendid mechanical properties (Patent Document 1). It has been strongly desired to develop a composite molded article formed of layers containing components having splendid features. There are some disclosures on a laminated film including a biodegradable resin (Patent Documents 2 and 3), but these documents substantially describe a laminated film produced by coextrusion injection of the same type of polyester and do not suggest the improvement in the adhesiveness at the interface.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-17153    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-144726    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-219487